Control of series-resonant inductive pickups

ABSTRACT

A series resonant inductive pickup has a bi-directional solid state switch in series with an inductor and a capacitor is controlled by switch control elements capable of causing the switching elements to repetitively be in either an open or a closed state, so that by varying the closed: open ratio of the switch the time-averaged amount of power picked up by the power pickup can be controlled. Switch is controlled by output of a voltage comparing circuit. A reference voltage source provides a basis for comparison of some fraction of a supply voltage in order to cause the switch to operate. The magnitude of capacitance in relation to the current drawn by the load determines the repetition rate of ON or OFF commands to the switch. If the current drawn by the load tends to zero, the proportion of time during which the switch is open will tend to 100%.

FIELD OF THE INVENTION

This invention relates to apparatus and methods for controlling thecurrent in a resonant secondary or pickup circuit for an inductive powertransfer circuit, capable of collecting electricity from a primarydistribution pathway having one or more conductors carrying alternatingcurrent.

BACKGROUND

In present day inductive power transfer (IPT) systems there is anenergisable trackway having at least one conductor (the primary), eachof which is surrounded by an alternating magnetic field during use Oneor more pickup devices, each of which includes at least one pickupwinding, form part of the system. Each is placed so as to intercept apart of the alternating magnetic field of the primary and thereby inducea useful current in the winding. Usually, the frequency of thealternating current in the primary is more or less matched to a resonantfrequency within the pickup. Practical supply frequencies range from amains frequency (50 Hz) up to commonly used frequencies in the 5-56 kHzrange, and as components capable of handling high power at higherfrequencies become available, the usable frequency may become higherGenerally, the supply frequency should be stable.

Resonant pickups may be either parallel-tuned or series-tuned in orderto improve the transfer of power Control of the power picked up from anIPT system has been a problem

The problem of control of power transfer might be solved by setting up asystem with a high capacity if uncontrolled and then either decouplingthe link between the primary pathway and the secondary pickup in someway, or by “wasting” excess power within the pickup circuit

Decoupling by interfering with the magnetic circuit itself could be doneby altering one or more of the dimensions of the gap, by adding orsubtracting permeable materials, by introducing a conductive block inwhich eddy currents may be generated, or (as a passive over-supplylimit) by incorporating a saturable ferri- or ferromagnetic element intothe magnetic circuit.

A related form of decoupling comprises changing the resonant frequencyof (usually) the pickup. Because this form can settle to a stablefrequency if the supply and load powers are stable, we regard it as aT=infinity configuration.

PRIOR ART

Many plans for IPT systems existed in the latter 19th century; forexample Tesla held a patent for powering a train using a high-voltagesystem with capacitative coupling, and a number of inventors filedpatents for at least telegraph message transfer by inductive meansacross a wide gap from a moving railway triage to a stationary trackway.

In the 20^(th) century there were many attempts to make commercial useof IPT systems, perhaps the most successful of these for larger powerapplications (e.g. to moving vehicles) is that of the Boys (the presentinventor).

Otto. GB1418128 (December 1974), described a series-tuned pickup havinga capacity suitable for use in powering a bus. Control of the powerpicked up was not included. Boys et al, in U.S. Pat. No. 5,293,308disclosed a parallel-tuned pickup control.

The problem to be solved is, to provide control over the transfer ofinductive power into any one pickup device to be at a level that matchesthe power being consumed If the transferred power is too small the loadis starved. If the transferred power is too large, the surplus currentcirculates within the pickup or over-supplies the load and may causedamage. Furthermore, surplus circulating current, by generating its ownfield, can block the onwards passage of primary power to other secondarycircuits sharing the same primary conductor.

Parallel-Tuned Pickup Control

Continuous (steady) control. Clearly, an absence of switchable controlelements is no control at all The saturable inductor of Boys et al asdescribed in NZ329195 (intended for control of overload or faultconditions rather than in normal usage) is also a form of continuouscontrol.

Per-cycle control, where the switching action is timed to occur in aspecified relationship with the phase of the circulating current in theresonant pickup, and occurs usually within every cycle:

Turner (assigned to Boeing) in U.S. Pat. No. 4,914,539) (Apr. 3, 1990)describes a regulator circuit in which a 38 kHz current is shorted outfor a variable duration per cycle, during a phase-related periodfollowing the moment when the voltage passes through zero with anegative slope. This is a regulator for inductively coupled power, for aspecific application (aircraft passenger seat entertainmentelectronics). In the example, semiconductor switching (to cause a shunt)occurs for a controllable period during each cycle. Any excess power issimply shunted to ground. This application exhibits a relatively smallvariation in load demand. For efficiency reasons this approach is notamenable to scaling upwards, particularly in situations in which theload requirements vary and may go down to zero. The semiconductors arerequired to work well at high frequency (low reverse recovery time is adesired feature).

Brooks (U.S. Pat. No. 5,045,770 or PCT/AU89/00035) may also be of thistype. Brooks describes a shunt circuit, integrated onto a single VLSIchip, for regulating power received from an alternating, looselycoupled, external magnetic field. The regulator shunts input power andincludes several modes of operation: diverting excess energy into aload, reducing the Q factor of the pickup circuit, and reducing thepower match to the load. A practical circuit includes a synchronousrectifier. This invention is not upwardly scalable.

Series-tuned Pickup Control

Continuous (steady) control has been described by Ehgtesadhi et al,within a number of publications in relation to a variable capacitorserving as the series-tuned capacitor, wherein the capacitor may beswitched through 64 steps from zero to slightly beyond the resonantcondition, so controlling the output from the more or less tuned pickup.

The saturable inductor of Boys et al as described in NZ329195 (intendedfor control of overload or fault conditions rather than in normal usage)is also a form of continuous control and could be used in aseries-connected resonant circuit.

Per-cycle control, where the switching action is timed to occur in aspecified relationship with the phase of the circulating current in theresonant pickup:

Pivnjak and Weiss in Elektrie vol 34 (1980), pp 339 to 341 describe a 5kHz series-resonant pickup having thyristor switching and (see FIG. 5)phase-related control means, together capable of varying the currentcirculating within the series-tuned pickup and hence of varying theoutput.

Lukacs B Nagy I, et al (Proceedings of the 4th Power Electronicsconference, Budapest, 1981) also describe at pages 83 to 92, aseries-resonant pickup having thyristor switching and phase-relatedcontrol means, together capable of varying the current circulatingwithin the series-tuned pickup and hence of varying the output.

OBJECT

It is an object of this invention to provide an improved pickup powercontrol system for inductive power transfer or at least to provide thepublic with a useful choice.

STATEMENT OF INVENTION

In a first broad aspect the invention provides for an inductive powertransfer system, a power pickup device with a series resonant circuitcomprising a pickup coil and a resonating capacitor selected so that thepickup is capable of resonance at a system-wide frequency, the powerpickup device further including power conditioning means capable ofconverting electricity that has been picked up into a conditioned formsuitable for consumption by a load, wherein apparatus capable ofcontrolling the amount of power picked up by the power pickup devicecomprises switching means in series with the pickup coil and in serieswith the resonating capacitor, together with switch controlling meanscapable of causing the switching means to repetitively be in either anopen or a closed state, so that by varying the respective proportion oftime that the switching means is either open or is closed thetime-averaged amount of power picked up by the power pickup device canbe controlled.

Preferably a repetitive cyclic operation of the switching means isrelatively slow, so that induced resonating currents may substantiallydie away during a normal “OFF” interval.

Note that switching rate drops as the size of the installation rises,and inductive power transfer installations capable of handling from lessthan one watt to perhaps one megawatt or more are known Typicalrepetition rates vary accordingly, from over 1 kHz to less than 100 Hz.Thus it is preferred that the switch control means is capable ofproviding a repetitive cyclic operation of the switching means which isinversely proportional to the amount of power to be collected by thepickup device.

A preferred switching means comprises a solid-state switch.

Preferably the switching means is a bidirectional switching meanscapable of controlling an alternating current.

Preferably the switching means is capable of carrying at least aresonating current of a usual magnitude circulating within the pickup.

One preferred solid-state switching means employs the type of deviceknown as an insulated gate bipolar transistor.

A more preferred solid-state switching means comprises a set of inverseparallel fast-recovery thyristors and an example switch device is anasymmetrical silicon-controlled rectifier (ASCR).

Preferably the switch controlling means is capable of responding to themagnitude of the conditioned power in a manner that tends to regulatethe magnitude of the conditioned power.

More preferably the switch controlling means is capable of responding tothe voltage of the conditioned power

Preferably the switch controlling means is also capable of responding tothe instantaneous voltage levels present at each side of the switchingmeans and hence causing the switching means to close at an instant whenthe the voltage levels present at each side of the switching means aresubstantially the same.

Preferably the switch controlling means is further capable of detectingthe current passing through the switching means and is capable ofdetermining when that current is at a zero crossing point, in order todetermine an instant which the switching means may be opened.

PREFERRED EMBODIMENT

The preferred embodiments to be described and illustrated in thisspecification are provided purely by way of example and are in no wayintended to be limiting as to the spirit or the scope of the invention.

DESCRIPTION OF FIGURES

FIG. 1: shows a simplified prior-art circuit diagram of a pickup controlmeans involving shorting the pickup circuit.

FIG. 2: shows a simplified circuit diagram of a pickup control means,including regulation means according to the invention.

FIG. 3: shows a circuit diagram of a pickup control means according tothe invention, including regulation means and means for determiningprecise control of timing of switch operations.

FIG. 4: shows a circuit diagram of a pickup control means according tothe invention, including a second mutually coupled coil in which theseries resonant coil is used for control and the non-resonant coil isused as a source of power.

FIG. 5: shows usage of a diode-protected pair of asymmetrical siliconcontrolled rectifiers (ASCRs) as an example switching means.

EXAMPLE 1

This invention relates to a secondary pickup having for control purposesa combination of major circuit components. See FIG. 1, in which thecircuit elements to the left of the rectifier R are: a pickup winding Wcapable of intercepting a magnetic field surrounding a primary inductiveconductor P a resonant circuit (of inductor W and series resonatingcapacitor B) are present at an input of the rectifier R. Note that C andS_(B) comprise elements of the prior art shortable parallel-tuned pickup(which lacks items B and S_(A)). They do not exist within the prototypeseries-tuned, controllable pickup.

A second circuit is located at an output of the rectifier R, generallyincluding a load L (which may be variable) and a smoothing capacitor D.S_(C) is an alternative position for a shorting switch for aparallel-tuned circuit. Most loads require a supply of DC, or sometimesof AC of a frequency other than that of the primary trackway (such asfor use by induction motors).

The general requirement is that the power transferred across the spaceshould be equal to the power consumed within the load (plus circuitwastage).

FIG. 2 illustrates a simple version of a controlled, series-resonantpickup within which the switch 203 is the equivalent of S_(A) in FIG. 1.Closing the switch completes the circuit and allows the output to rise.Opening the switch interrupts the series circuit, stops power delivery,and halts resonance. This switch is controlled by output 214 of avoltage comparing circuit 212, here an operational amplifier havingpositive feedback according to the ratio of the values of 210 to 211 inorder to implement some hysteresis. The preferred control method forthis control means is hard on/hard off so that resistive losses areminimised, and a closed: open ratio is selected so that the outputvoltage is kept at about a desired amount. Times are generallyequivalent to tens or hundreds of cycles. A reference voltage source 213provides a basis for comparison of some fraction of a supply voltage(208) in order to cause the switch to operate. In this case, themagnitude of capacitance D) in relation to the current I_(O) drawn bythe load L determines the repetition rate of ON or OFF commands to theswitch. Preferably the repetition rate is of the order of 10 to 30milliseconds and the mark:space ratio, which comprises the effect ofregulation is determined at any time by the current provided to the loadL in relation to the current obtained from the primary pathway P.Clearly, if the current I_(O) drawn by the load L tends to zero, theproportion of time during which the switch 203 is open will tend to100%.

The control means 203 is required to carry the resonating current, towithstand the likely peak open-circuit voltage and any transients, andshould be bidirectional. The control means is a bidirectional switch andmay be constructed using a variety of solid-state devices as is known inthe art.

Preferably the bidirectional switch 203 is placed in the AC side of thecircuit, preceding the rectifier, because of the possibility thatotherwise high-voltage transients may cause breakdown of the componentsof the rectifier, although a unidirectional switch could be placedbetween rectifier 204 and line 208. In fact, we believe that if theswitch is opened at other than a zero-crossing interval with respect tocurrent, any remaining flux about the pickup coil will collapse into thewindings and that resultant energy will then be dissipated either withinthe switch or within protective (snubber) devices.

Opening the switch 203 results in (1) destruction of the condition ofresonance, and (2) interruption of the connection between the voltagesource and the load, so that the output, as presented to the load Lafter rectification by rectifier module R, and smoothing by capacitor D,falls.

The output is a substantially constant voltage, the limits of itsexcursions being determined during normal operation by the hysteresiswithin the controller 207 and by the time taken to resume charging afterthe control means 203 is again closed. Alternatively a current-sensitivetransducer configuration may be used to provide a substantiallyconstant-current output. In this case a sense resistor or a magneticdevice capable of sensing a current in a wire (LEM, Hall effect deviceor the like) provides a magnitude-sensitive input.

FIG. 3 shows methods for coping with some of the disadvantages of usinga series-connected switch. The more practical circuit 300 includes theblock 207, and also includes logic and sensing means, intended to causethe switch to delay opening until a moment occurs when zero current isdetected in the adjacent wiring, and to delay closing until a momentoccurs in which each of the opened switch contacts has the same voltageupon it.

A voltage comparison means 302 is connected across the switch 203 andits output, presented to logic circuitry, goes through a transition whenthe inputs become equal, The logic circuitry 301 comprises a circuitarrangement, well known to those skilled in the art, capable of delayingthe forwarding (through control line 304) of a “CLOSE” command emanatingfrom the block 207 through line 214 to the switch until such time as theoutput of 302 exhibits a transition. The logic circuitry 301 is alsocapable of delaying the emission of an “OPEN” command to the switch 203until such time as the output of current sensor 305 indicates that zerocurrent is detected in the adjacent wiring. The block 301 may compriseconventional logic devices, or a PAL or PLD logic circuit andappropriate preferably isolated drivers for the switch 203. In practice,example switching means might be a fast electromechanical relay, butmore preferably a solid-state device such as a “TRIAC”, or aback-to-back pair of unidirectional switching devices such as thyristors(also known as silicon controlled rectifiers), power FETs, IGBTs, or thelike, along with suitable anti-transient protection means. Of course,novel devices suitable for this application may be produced in thefuture.

The additional synchronising equipment (sensors and logic) is preferredin order to increase the working life of the switching devices, and toprevent large and possibly damaging transients from being created withinthe secondary circuit, which transients may also be propagated into theprimary line and, as interference, into the environment.

EXAMPLE 2

FIG. 4 illustrates an optimised pickup arrangement suitable for examplein the charging of batteries in which a first, high-voltage winding 401is provided with a series tuned resonating capacitor 202 and a switch203 capable of opening or closing fee circuit as described previously inthis section. A second, low-voltage circuit comprises a simple coil 402(the windings of which may in practice comprise a single turn) directlywired to a rectifier 204 which in turn feeds a battery module ormonobloc 406. Control of the circuit (through block 207 and optionallyalso a block like 301 (details not shown)) may be either by supplied ordrawn current or by voltage measurements of the across-battery voltage(or both). Coils 401 and 402 share a common core and when 401 is unableto resonate, the output of 402 is substantially reduced. Thisconfiguration has the advantage that the series switch is not requiredto interrupt a high current. Heavy current devices are more expensivethan high voltage devices and heavy current devices would developgreater wastage. There are significantly reduced losses if a seriesswitching device is used, as compared to a parallel-tuned, shortedswitch option. This type of circuit is particularly suited to slowswitching cycles of the order of 20 to 50 Hz whereas the supplyfrequency may be of the order of 15 kHz. Slow switching allows time forthe resonant oscillations in the pickup to substantially stop.

FIG. 5 shows usage of a diode-protected pair of asymmetrical siliconcontrolled rectifiers (ASCRs) as a convenient way to provide, for theinvention, a switching means 203 which will operate at zero-crossingmoments. The ASCR device as a class is no longer widely used as a resultof its poor inverse voltage rating (only 5 V or so causing damage).However it commutates, or turns off in the event of a reversal ofapplied voltage, quickly and typically within a microsecond. This iswell within the 50 microsecond duration of a half-cycle at 10 kHz. Hencea configuration such as that shown in FIG. 5 simply has to inhibit thegate drive somewhere within the nearest preceding half-cycle and thedevice itself will switch off at the zero-crossing point. It would beless easy to employ a discrete current sensing means, logic, and then tocontrol switching devices of the generic power FET type, which lack theself-commutation feature. FIG. 5 includes two ASCRs (501, 502) and oneprotective diode (503, 504) across each one, with gate control inputs505, 506 supplied appropriately (such as with an isolated input whichmay well comprise a DC current source powered across an air gap by acontrolled (switched) high frequency supply). Other than ASCRs, selectedfast-recovery thyristors may also be used.

VARIATIONS

Variations of this principle of inductive power collection control mayinvolve selection of alternative forms of switching devices includingdevices not known at the time of filing this specification.

Improved circuitry for the control of output may sense additionalcurrents or voltages, and may also be susceptible to external control asby a human driver or from automatically generated commands.

While the actual mark:space ratio is usually set by the demand forpower, in relation to the amount of power collected by the pickup whencollecting, the repeat frequency of mark:space ratios may be varied overa wide range in order to optimise efficiency, device lifetime,interactions with other units, and interference The repetition rate ofthe invention may be as fast as once every cycle or two of the supplyfrequency or as slow as many hundreds of cycles, depending to someextent on the excursions of output voltage that can be tolerated by aparticular kind of load.

The configurations described in this specification may be adaptedtowards higher speed control but switching losses may become significantand unless multiples of single cycles are switched, which provides largeincrements of control, significant transients may be generated when theswitch is forced to operate irrespective of phase.

COMMERCIAL BENEFITS OR ADVANTAGES

There are a number of situations in which a series-resonant pickupoffers more suitable forms of electric power than does a parallelresonant pickup. The invention, a control means for a series-resonantpickup loop which is capable of minimising the amount of currentcirculating within the pickup inductor, provides for the simultaneousand non-conflicting use of more than one pickup device sharing the samepowered section of primary inductive pathway. Hitherto, it would bedifficult to use more than one pickup device because of the opposingeffect of high levels of circulating secondary current upon a primarycurrent.

Although several preferred examples of the invention as described abovehave been disclosed for illustrative purposes, those skilled in the artwill appreciate that various modifications, additions, and substitutionsare possible without departing from the scope of the invention as setforth.

What is claimed is:
 1. An inductive power pickup device having a seriesresonant circuit comprising a pickup coil and a resonating capacitorselected so that the pickup is capable of resonance at a system-widefrequency, the power pickup device further including power conditioningmeans capable of converting electricity that has been picked up into aconditioned form suitable for consumption by a load,characterised-in-that apparatus capable of controlling the amount ofpower picked up by the power pickup device comprises switching means inseries with the pickup coil and in series with the resonating capacitor,together with switch controlling means causing the switching means torepetitively be in either an open or a closed state, so that by varyingthe respective proportion of time that the switching means is eitheropen or is closed the time-averaged amount of power picked up by thepower pickup device can be controlled, the pickup coil positionedadjacent a primary inductive conductor for intercepting a magnetic fieldsurrounding the primary inductive conductor.
 2. An inductive powerpickup device as claimed in claim 1, characterised-in-that the switchcontrol means is capable of providing a repetitive cyclic operation ofthe switching means which is inversely proportional to the amount ofpower to be collected by the pickup device.
 3. An inductive power pickupdevice as claimed in claim 1, characterised-in-that the switching meansis a solid-state switching means that comprises a set of inverseparallel fast-recovery thyristors.
 4. An inductive power pickup deviceas claimed in claim 1, characterised-in-that the switching means is anasymmetrical silicon-controlled rectifier (ASCR).
 5. An inductive powerpickup device as claimed in claim 1, characterised-in-that the switchcontrolling means is capable of responding to the magnitude of theconditioned power in a manner that tends to regulate the magnitude ofthe conditioned power.
 6. An inductive power pickup device as claimed inclaim 1, characterised-in-that the switch controlling means is alsocapable of responding to the instantaneous voltage levels present ateach side of the switching means and hence causing the switching meansto close at an instant when the voltage levels present at each side ofthe switching means are substantially the same.
 7. An inductive powerpickup device as claimed in claim 6, characterised-in-that the switchcontrolling means is further capable of detecting the current passingthrough the switching means and is capable of determining when thatcurrent is at a zero crossing point, in order to determine an instant atwhich the switching means may be opened.
 8. The pickup device of claim1, wherein the switch controlling means comprises: a switching ratedetermining capacitor connected across an output of the powerconditioner means; a voltage comparing circuit having on outputconnected to the switching means to control the opening and closing ofthe switching means; a reference voltage connected as a first input tothe voltage comparing circuit; and a fractional voltage of an outputsupply voltage of the power conditioner means connected as a secondinput to the voltage comparing circuit, wherein the voltage comparingcircuit compares the reference voltage to the fractional voltage of theoutput supply voltage, and a capacitance of the switching ratedetermining capacitor, in relation to a current drawn from the powerconditioner means by a load, determines a repetition rate of opening andclosing of the switch.
 9. An inductive power pickup device having aseries resonant circuit comprising a pickup coil and a resonatingcapacitor selected so that the pickup is capable of resonance at asystem-wide frequency, the power pickup device further including powerconditioning means capable of converting electricity that has beenpicked up into a conditioned form suitable for consumption by a load,characterized in that an apparatus capable of controlling the amount ofpower picked up by the power pickup device comprises switching means inseries with the pickup coil and in series with the resonating capacitor,together with switch controlling means capable of causing the switchingmeans to repetitively be in either an open or a closed state, so that byvarying the respective proportion of time that the switching means iseither open or is closed the time-averaged amount of power picked up bythe power pickup device can be controlled and wherein the switchcontrolling means is capable of responding to the voltage of theconditioned power.
 10. An inductive power pickup device as claimed inclaim 9, characterized in that the switch controlling means is capableof providing a relatively slow repetitive cyclic operation of theswitching means, so that induced resonating currents may substantiallydie away during a normal “OFF” interval.
 11. An inductive power pickupdevice as claimed in claim 10, characterized in that the switch controlmeans is capable of providing a repetitive cyclic operation of theswitching means which is inversely proportional to the amount of powerto be collected by the pickup device.
 12. An inductive power pickupdevice as claimed in claim 10, characterized in that the switching meanscomprises a bi-directional solid-state switching means capable ofcontrolling an alternating current.
 13. An inductive power pickup deviceas claimed in claim 12, characterized in that the solid-state switchingmeans comprises a set of inverse parallel fast-recovery thyristors. 14.An inductive power pickup device as claimed in claim 12, characterizedin that the switching means is an asymmetrical silicon-controlledrectifier (ASCR).
 15. An inductive power pickup device as claimed inclaim 10, characterized in that the switch controlling means is capableof responding to the magnitude of the conditioned power in a manner thattends to regulate the magnitude of the conditioned power.
 16. Aninductive power pickup device as claimed in claim 9, characterized inthat the switch controlling means is also capable of responding to theinstantaneous voltage levels present at each side of the switching meansand hence causing the switching means to close at an instant when thevoltage levels present at each side of the switching means aresubstantially the same.
 17. An inductive power pickup device as claimedin claim 9, characterized in that the switch controlling means isfurther capable of detecting the current passing through the switchingmeans and is capable of determining when that current is at a zerocrossing point, in order to determine an instant at which the switchingmeans may be opened.
 18. The pickup device of claim 9, wherein theswitch controlling means comprises: a switching rate determiningcapacitor connected across an output of the power conditioner means; avoltage comparing circuit comprising an operational amplifier having onoutput connected to the switching means to control the opening andclosing of the switching means; a reference voltage connected as a firstinput to the operational amplifier; and a fractional voltage of anoutput supply voltage of the power conditioner means connected as asecond input to the operational amplifier, wherein, a capacitance of theswitching rate determining capacitor, in relation to a current drawnfrom the power conditioner means by a load, determines a repetition rateof opening and closing of the switch.
 19. An inductive power pickupdevice, comprising: a series resonant circuit comprising a switch inseries with a pickup coil positioned for intercepting a magnetic fieldsurrounding a primary inductive conductor, the pickup coil in serieswith a resonating capacitor, the capacitor selected to provide a pickupcapable of resonance at a system-wide frequency; a power conditionerconnected across the pickup coil and capacitor for convertingelectricity that has been picked up by the series resonant circuit intoa conditioned power, the conditioned power being at an output of thepower conditioner at a supply voltage and in a form suitable for loadconsumption; a switch controlling circuit connected to open and closethe switch, the switch controlling circuit being responsive to thesupply voltage of the conditioned power in opening and closing theswitch, the switch controlling circuit repetitively opening and closingthe switch to vary a time that the switch is open so that atime-averaged amount of power picked up by the series resonant circuitis controlled.
 20. The pickup device of claim 19, wherein the switchcontrolling circuit comprises: a switching rate determining capacitorconnected across the output of the power conditioner; a voltagecomparing circuit having on output connected to the switch to controlthe opening and closing of the switch; a reference voltage connected asa first input to the voltage comparing circuit; and a fractional voltageof the supply voltage of the conditioned power connected as a secondinput to the voltage comparing circuit, wherein the voltage comparingcircuit compares the reference voltage to the fractional voltage of thesupply voltage as a basis for controlling the opening and closing of theswitch, and a capacitance of the switching rate determining capacitor,in relation to a current drawn from the power conditioner by a load,determines a repetition rate of opening and closing of the switch.